Subject:

Abstract:

Metacommunity theory suggests a potentially important
role for dispersal in diversity maintenance at local, as well as regional,
scales. In addition, propagule addition experiments have shown that
dispersal often limits local diversity. However, actual dispersal rates
into local communities and the contribution of immigrants to observed
local diversity are poorly known. We present a new approach
that partitions the diversity of a target community into dispersalmaintained
and dispersal-independent components. Specifically, we
quantify distances through space and time to the nearest potential
seed source for naturally occurring recruits in target communities
by using hierarchical data on species pools (local, site, region, and
seed bank). Using this “recruit tag” approach, we found that dispersal
contributed 29%–57% of the seedling diversity in perennial grasslands
with different successional histories. However, both dispersal
and seedling mortality remained remarkably constant, in absolute
terms, over succession. The considerable loss of diversity over secondary
succession (66%), therefore, could be understood only by
considering how these processes interact with the decreasing disturbance
rate (i.e., frequency of gaps) in later-successional sites. We
conclude that a metacommunity perspective is relevant and necessary
to understand the diversity and community assembly of this study
system.